I think there is one seemingly insignificant difference with big (behavioral) consequences: mails need to be "opened", and they're "unread" until you do that. T

I think there is one seemingly insignificant difference with big (behavioral) consequences: mails need to be "opened", and they're "unread" until you do that. This gives you an option, even after you learn what the email is about (reading the title, the sender, and knowing some context), whether you want to read it now or not. On Slack, like any other instant messaging (especially the ones where the fact that you've seen a message is communicated to the other side), you don't have this option. You can, of course, snooze messages, but I doubt many people even know about that option, and a high fraction of those who do know just find it, at that moment, easier to answer immediately then to think about "scheduling" that answer. Combining this with other characteristics that make online chat more like in-person chat (from simple functions like the ease of sending messages by pressing Enter, to more advanced ones like seeing when the other side is typing), it shifts the behavior, and people feel compelled to respond instantly - both by the expectations set by others ("everyone responds instantly to Slack so I have to as well") but maybe even more importantly by subconscious "burden" of not responding to an "already opened message".